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What is the use of promises in javascript?

Answer»

Promises are used to handle asynchronous operations in javascript.

Before promises, callbacks were used to handle asynchronous operations. But due to the limited functionality of callbacks, using multiple callbacks to handle asynchronous code can lead to unmanageable code.

Promise object has four states -


  • Pending - Initial state of promise. This state represents that the promise has neither been fulfilled nor been rejected, it is in the pending state.

  • Fulfilled - This state represents that the promise has been fulfilled, meaning the async operation is completed.

  • Rejected - This state represents that the promise has been rejected for some reason, meaning the async operation has failed.

  • Settled - This state represents that the promise has been either rejected or fulfilled.

A promise is created using the Promise constructor which takes in a callback function with two parameters, resolve and reject respectively.

resolve is a function that will be called when the async operation has been successfully completed.

reject is a function that will be called, when the async operation fails or if some error occurs.

Example of a promise:

Promises are used to handle asynchronous operations like server requests, for ease of understanding, we are using an operation to calculate the sum of three elements.

In the function below, we are returning a promise inside a function:

function sumOfThreeElements(...elements){
return new Promise((resolve,reject)=>{
if(elements.length > 3 ){
reject("Only three elements or less are allowed");
}
else{
let sum = 0;
let i = 0;
while(i < elements.length){
sum += elements[i];
i++;
}
resolve("Sum has been calculated: "+sum);
}
})
}

In the code above, we are calculating the sum of three elements, if the length of the elements array is more than 3, a promise is rejected, or else the promise is resolved and the sum is returned.

We can consume any promise by attaching then() and catch() methods to the consumer.

then() method is used to access the result when the promise is fulfilled.

catch() method is used to access the result/error when the promise is rejected. In the code below, we are consuming the promise:

sumOfThreeElements(4, 5, 6)
.then(result=> console.log(result))
.catch(error=> console.log(error));
// In the code above, the promise is fulfilled so the then() method gets executed

sumOfThreeElements(7, 0, 33, 41)
.then(result => console.log(result))
.catch(error=> console.log(error));
// In the code above, the promise is rejected hence the catch() method gets executed


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